1. Field of the Invention
The invention relates to television tuners, and more particularly, to a double conversion television tuner using a harmonic mixer architecture with no second intermediate frequency to process a received RF signal.
2. Description of the Prior Art
One of the most significant costs in television manufacturing is the cost of the tuner. The typical cost of a television (TV) tuner is in the neighborhood of $15.00, which, relative to the cost of the entire television set, is very substantial. Furthermore, with the increasing desire to integrate TV functions into personal computer (PC) systems and other electronic devices, the cost of the tuner needs to be reduced. Part of the solution to reducing tuner cost is to reduce the number of components in the tuner.
Traditionally, tuners have been comprised of two basic components. The first component performs high frequency to intermediate frequency (RF to IF) conversion. Subsequently, the second component performs IF to baseband conversion. The TV tuner was originally designed for broadcast television reception within a television set, which is essentially a stand-alone unit containing a cathode ray picture tube. As such, TV tuners were originally integral parts embedded in a single-purpose device.
Presently, however, state-of-the-art consumer electronic devices use TV tuners that are not a built-in part of a television set. The tuner is a separate element that is connected to a cathode ray picture tube at some point, but the tuner is not an integral part of the monitor. As previously mentioned, TV tuners may be fabricated on circuit boards and then installed in personal computer systems, thereby allowing the PC to function as a television set. These tuners convert a radio frequency television signal into a baseband (or low frequency) video signal, which can then be passed on to other elements in the PC for video processing applications.
The circuit component that performs the RF-to-IF conversion typically comprises one or two integrated circuits and numerous discrete elements such as inductors, capacitors and/or transistors. The IF-to-baseband conversion typically includes another integrated circuit, several filter elements, such as ceramic filters and SAW filters, a series of tuning and control elements, such as resistors and potentiometers, variable inductors and/or capacitors, and some other additional external components. Thus, the complexity of the tuner is fairly high and typically there may be between 100 and 200 elements on a circuit board. Furthermore, state-of-the-art TV tuners still require that each tuner be aligned by manual tuning before leaving the factory. This manual tuning is one of the most expensive costs associated with the manufacturing process and an important factor in the cost of tuners.
Broadcast television tuners of the past have gone through an evolution over a period of more than 60 years. The earliest tuners utilized vacuum tube technology and required that the minimum number of vacuum tubes possible be used due to their cost, power consumption and dimensions. Therefore, passive components, such as resistors, capacitors, inductors and transformers, were used as much as possible in most designs. This style of design continued until about 1960 when TV tuner components, particularly vacuum tubes began to be replaced by bipolar and MOS transistors. However; the active device count still defined the cost and size limits of TV tuners and active device count minimization continued.
In the early 1970's the integrated circuit became viable as an element in the television tuner and the design techniques were dramatically changed. Many functions of the tuner utilizing only one tube or transistor were being replaced with 4 to 20 individual transistors which could perform the same function with better precision, less space, less power, less heat generation, and lower cost. The introduction of the integrated circuit was gradual, first encompassing only low frequency elements and then eventually high frequency active elements. Nonetheless, many passive elements external to the integrated circuits remained in TV tuner designs.
One advance, the SAW (surface acoustic wave) filters made a significant change in that several manually tuned inductors and capacitors could be removed from the tuners and receive-filtering performance could be improved within a much smaller space and at reduced cost. However, the SAW filter, which is fabricated on a ceramic substrate, cannot be integrated on a silicon wafer with the rest of the active circuitry and must therefore remain a discrete component in the final design. The trend of the 1980's was to miniaturize all of the passive components and simplify their associated manual tuning at the factory. In recent years, TV tuners have been reduced in size from requiring fairly large enclosures, about 2″×5″×1″, to much smaller enclosures, about ½″×2″×⅜″. There is a high premium placed on small size because TV tuners are being used in smaller and smaller computers, television sets, and VCRs. As the equipment in which tuners are used becomes smaller, the size of the TV tuner must also decrease.
As the size of the tuner is reduced, and as tuners are used in a wider variety of devices, cost becomes more critical and must be reduced as much as possible in order not to represent a large portion of the final product cost. When a tuner is used in a television set, the tuner size is less critical because the television set inherently has a large mass. But when a tuner is used in other electronic equipments, space becomes a premium and the footprint of the tuner becomes critical.
FIG. 1 shows a highly integrated television tuner 100 on a single microcircuit as disclosed by U.S. Pat. No. 5,737,035. The television tuner 100 includes an adjustable low noise amplifier 101, a first mixer 102, a first local oscillator 104, a band-pass filter 106, a second mixer 108, being an image rejection type mixer, a second local oscillator 110, a first intermediate frequency amplifier 112, a second band-pass filter 114, and a variable intermediate frequency amplifier 116. However, as the television tuner 100 requires the use of a special image rejection mixer for the second mixer 108, the cost of the tuner is increased. Additionally, the first local oscillator 104 is used in conjunction with the first mixer 102 to up-convert a particular channel selected from an incoming RF signal. This means the first local oscillator 104 must be a variable frequency local oscillator having a large operating frequency range. Because the phase noise over the operating frequency range of the first local oscillator 104 must meet a specific phase noise requirement, typically 84 dBC/Hz, a plurality of VCOs having smaller frequency ranges, and therefore lower phase noise, must be used. This again increases the complexity and cost of the television tuner 100. Accordingly, a need exists for a tuner not having these requirements in order to reduce the cost.